A fuel cell system includes a fuel cell stack, a fuel gas supply system for supplying a fuel gas to the fuel cell stack, and an oxidant gas supply system for supplying an oxidant gas to the fuel cell stack. The fuel gas supply system includes, for example, a tank for storing the fuel gas under high pressure, and a fuel gas passage connecting the tank with the fuel cell stack. The fuel gas supply system typically includes a first pressure reducing valve disposed on the upstream side of the fuel gas passage and a second pressure reducing valve disposed on the downstream side of the fuel gas passage. Each of the pressure reducing valves has the function of adjusting the pressure at the downstream side thereof to a target pressure.
Now, in the event of a sudden decrease of the amount of fuel gas consumed by the fuel cell stack, typically, the second pressure reducing valve on the downstream side closes first, and the first pressure reducing valve disposed on the upstream side closes thereafter. Specifically, once the amount of fuel gas consumed by the fuel cell stack decreases suddenly, pressure rises on the downstream side of the second pressure reducing valve, and as a result the second pressure reducing valve closes. Once the second pressure reducing valve begins to close, pressure rises on the downstream side of the first pressure reducing valve, and as a result the first pressure reducing valve closes. Where as described above, the second pressure reducing valve is set to the closed state first and the first pressure reducing valve is set to the closed state thereafter, pressure rises excessively between the two pressure reducing valves. Thus, in the background art, there is a risk of damage to the two pressure reducing valves and to the passage between the two pressure reducing valves, and it is necessary to improve pressure resistance of these portions.